An x-ray xerographic photoreceptor having a high arsenic doped selenium alloy layer near the aluminum or transparent substrate interface to provide a hole blocking layer for negative charging of the photoreceptor.
When a negative charge is placed on the top surface of a photoreceptor and the substrate acquires a positive counter-charge, positive charges or holes, enter the selenium layer from the substrate. In conventional paper transfer systems, negative charging commonly occurs when the photoreceptor passes under a negative corotron at the image transfer station. Negative charging at transfer is described in U.S. Pat. No. 5,023,661 which is incorporated herein by reference.
FIG. 1A of the cited patent shows the result of a transfer corotron charging the top surface of the photoreceptor to a negative potential. The substrate is held at ground potential so that an electrostatic field is developed between the photoreceptor substrate and the top surface overcoating in response to this applied field, holes migrate upward from the substrate toward the amorphous selenium layer. If the surface of the selenium layer which is in contact with the substrate has a selenium crystallite defect then holes will enter the selenium at this site and will migrate under the influence of the field into the upper-most portion of the selenium photoconductor. These concentrations of migrated charge at points over crystallite defects ultimately generate artifacts in subsequent image cycles. The solution proposed by the referenced patent is to pre-charge the photoreceptor to remove these positive charge concentrations immediately before the beginning of a standard image cycle.
The cited reference also shows that the top surface of the selenium bulk layer has an arsenic-rich layer to increase its hardness and to thereby prevent scratching and surface crystallization. The photoreceptor is operable and durable without this top surface layer. However, if the manufacturer prefers, he has the option of applying a protective layer to the photoreceptor for additional surface durability. The arsenic increases the hardness of the selenium by raising the alloy viscosity, glass transition temperature and boiling points.
Because the positive charges tend to concentrate above crystallite sites creating defects, two other commonly assigned U.S. Pat. No. 5,300,784 and 5.320,927, describing how the formation of these crystallites can be prevented, are incorporated herein by reference. During the manufacture of xerographic photoreceptors, the selenium-arsenic alloy is retained in a crucible array in the vacuum deposition chamber and is evaporated under vacuum onto the substrate in a molten liquid form. Later, as the selenium cools and solidifies, the selenium at the substrate interface may crystallize if insufficient arsenic is present. The arsenic tends to break up the regularity of the material and increases its viscosity at a given temperature making the formation of crystallites less likely. Because arsenic tends to fractionate last from an arsenic-selenium alloy during vacuum deposition, it is difficult to achieve high arsenic concentration in the initially deposited layer as desired. This initial layer is therefore deposited from a separate crucible array in a thicknes range of 0.05 to 5 microns. Minimization of crystallites prevents local concentrations of positive charge and thereby minimizes the appearance of point discharge artifacts under negative charging conditions. This intermediate layer is commonly not used in most production operations but may be used at the manufacturer's discretion, particularly in instances where interface-related artifacts pose a problem. When this intermediate layer is not used, the bulk layer is deposited directly on the substrate.
Even in the absence of interface crystallites, positive charge tends to inject into the selenium layer more or less uniformly contributing to rapid dark decay thereby preventing the retention of a negative surface charge.
In a system that utilizes positive surface charging, the substrate is negative with respect to the selenium top surface and holes, therefore, are not generated at the substrate interface during the charging cycle. The dark decay is low and the photoreceptor retains its initial surface charge and resultant initial electrostratic field. This internal field is essential for the generation and transport of charge during imaging. During negative transfer following exposure, the top surface negative potential is small, its application brief so that the presence of holes at the substrate interface is not significant. However, if the initial surface charge is negative, the generation of holes from the substrate will discharge the negative charge at the top surface creating dark decay and rendering the plate unsuitable for exposure. For this reason, selenium xerographic copier systems use positive charging and rely on the migration of positive charge during exposure to form an image.
For medical imaging, which includes mammography as well as radiography, there is a need to expose the patient to the lowest possible x-ray dose. Patient-dose minimization, moreover, is essential regardless of whether the development mode utilizes conventional powder, liquid or digital technologies. In a line copier, the visible light radiation does not penetrate beyond a few microns of selenium and therefore, only holes migrate down from the top surface to the substrate to discharge the photoreceptor Therefore, the selenium layer need not be so thick as in medical imaging, fifty to sixty microns being typical for copier applications. However, for medical imaging, x-rays penetrate to a greater depth. Therefore, to capture as many radiation quanta as possible, x-ray photoreceptors are thicker; 150 to 450 microns is a typical thickness range. In medical photoreceptors, electron-hole pairs are created throughout the bulk of the material. Therefore, to obtain the greatest electrostatic response from a unit of x-radiation, the migration of both electrons and holes is used to discharge the photoreceptor.
In certain powder, liquid and digital imaging systems configuration design requirements arise in which it is advantageous to be able to charge a plate negatively or positively while preventing the migration of holes from discharging the plate before it can be used to generate an image.